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STUDY FOR SAFETY AT A RELATIVELY SHORT TUNNEL WHEN
A TUNNEL FIRE OCCURRED
Y. Mikame1,2, N. Kawabata1, M. Seike1, M. Hasegawa1
1
Kanazawa University, Japan
2
Metropolitan Expressway Company Limited, Japan
ABSTRACT
Progress in vehicle exhaust control has greatly improved environments in road tunnels in
recent years. Consequently, we discuss the downsizing or removal of tunnel ventilation
systems formerly installed to secure the environments in road tunnels. On the other hand,
tunnel ventilation systems are also used as smoke control systems in case of fires. So it is also
important to discuss safety from tunnel fires. We studied factors that affect the safety of
tunnel users in a relatively short road tunnel (around 500m, without a ventilation system)
when a tunnel fire has occurred. The examination was analysed by a three-dimensional
simulation (Large-Eddy Simulation model) to reproduce the smoke spread and by a onedimensional evacuation simulation. We evaluated the number of people requiring help
(NPRH) who cannot evacuate from a tunnel fire. Among the results of the study without a
ventilation system, we found some conditions related to safety when a tunnel fire has
occurred. The conditions were the fire source point, road longitudinal gradient, presence or
absence of wind velocity and a bus. Especially when evacuating from a bus, people need more
time to evacuate. This case has an increased risk of someone being left behind in the bus. And
we confirmed that many passengers cannot evacuate from a bus because of the smoke from
the fire when the bus is close to the fire source point. The second factor influencing safety is
the natural wind in the tunnels. Even in a small natural wind of about 1.0 m/s case, smoke
catches up to tunnel users when the direction of the natural wind matches the evacuation
direction. In this case, we have confirmed that many evacuees cannot evacuate, even there is
no bus.
Keywords: tunnel fire, short tunnels, natural ventilation, bus, evacuation simulation
1.
INTRODUCTION
In Japan, the scale of tunnel ventilation systems has been determined by the amount of
vehicles exhausting gases, and this system is used when a tunnel fire has occurred. Tunnel
ventilation systems have been installed not only in long distance tunnels, but also in short
distance tunnels with heavy traffic in urban areas. The Tokyo metropolitan expressways carry
heavy traffic, so its operators put tunnel ventilation systems in tunnels about 300m long.
Lately, the improvement of vehicle exhaust control has greatly improved the environments of
road tunnels. For this reason, this system does not ventilate vehicle exhausts, but operates
when a tunnel fire has occurred. Long tunnels about 10km are designed to prepare for a tunnel
fire [1], this is a dangerous condition. In contrast, in relatively short tunnels of about 500m,
the safety of tunnel users has almost never been studied. For tunnel ventilation systems, the
central issue is downsizing and removal. This study determined factors affecting the safety of
tunnel users when removing a tunnel ventilation system from a relatively short road tunnel.
7thInternational Conference ‘Tunnel Safety and Ventilation’ 2014, Graz
- 134 2.
EXAMINATION CONDITIONS
2.1. Simulation model
The spread of smoke from a tunnel fire was simulated using an original three-dimensional
simulation (Fireles) [2]. This simulation is developed partly by the authors, and the turbulence
model is a Large-Eddy-Simulation. Comparing this with full size tunnel experiment results
confirmed the accuracy of this simulation [3]. In Japan, it is generally used to study road
tunnel safety when a tunnel fire has occurred. To grasp the simulation of the evacuation of
tunnel users, this study used an evacuation simulation [4]. This simulation calculated the
number of people requiring help (NPRH), those who cannot evacuate for 10 minutes after the
occurrence of the tunnel fire.
2.2. Specification of the model tunnel
•
•
•
•
Tunnel length
Cross section of the tunnel
Longitudinal gradient
The computational grid sizes
•
Number of divisions
450m
rectangular (around 8.5 m W x 4.5m H)
-4% - 4%
0.33 m in the x direction,
0.31 m in the y direction, 0.23m in the z direction
1429 in the x direction, 43 in the y direction,
29 in the z direction
(Include area outside the tunnel)
Tunnel entrance
Tunnel exit
x
Fire source point
225m
130m
150m
150m
-4%
150m
4%
0%
Figure 1: Outline of the tunnel that was studied
2.3. Tunnel fire conditions
When a tunnel fire has occurred, factors affecting the safety of tunnel users are fire source
points, natural wind, and arrangements and configuration of vehicles. The fire source points
have an impact on smoke spread by the longitudinal gradient. The natural wind, which is the
pressure difference between the tunnel entrance and exit for natural ventilation (Δ), causes the
smoke to spread in the tunnel. The arrangement and configuration of vehicles is related to the
number of NPRH. Table 1 shows fire source points and pressure difference Δ conditions.
Vehicles were arranged from the tunnel entrance to the fire source point and from the fire
source point to the tunnel exit. The configuration of vehicles assumed only passenger vehicles,
large size vehicles comprised 10% of total traffic and a bus was included. In this paper, it was
assumed that a single large size vehicle caught fire in the tunnel. Heat release rate adopted
was 30MW, and it changes over time to become constant after 480 seconds.
7thInternational Conference ‘Tunnel Safety and Ventilation’ 2014, Graz
- 135 Table 1: Tunnel fire conditions
Category
Conditions
x=225m (Sections of longitudinal gradient 0% )
x=130m (Sections of longitudinal gradient -4%)
Fire source points
Δ (Pressure difference between the tunnel
entrance and exit for natural ventilation)
0Pa [0 m/s]，5Pa [0.85 m/s]，10Pa [1.2 m/s]
Table 2: Number of passengers (per vehicle)
Number of passengers
Passenger vehicles
Average of 1.4 people
Large size vehicles
Average of 1.3 people
Bus
50 people
Conditions under which it is difficult for tunnel users to evacuate were defined as smoke
density greater than Cs 0.4 [1/m] and smoke height less than 1.5 m from road surface. Smoke
density of Cs 0.4 [1/m] reaching the ceiling triggered the start of evacuation of tunnel users;
they evacuate when they see other tunnel users evacuating. The evacuation velocity
distribution linearly increased from 0.9 m/s to 1.2 m/s, was constant from 1.2 m/s to 1.8 m/s,
and linearly decreased from 1.8 m/s to 2.1 m/s. The direction of evacuation of tunnel users
was assumed to be evacuation towards the tunnel portals.
3.
TRENDS IN CONFIGURATION AND PLACEMENT OF VEHICLES
3.1. Simulation conditions
Table 3 shows simulation cases, excluding cases including emergency exits. These cases are
mentioned later. A bus is located 100 m from the tunnel entrance and exit.
Table 3: Simulation cases
Number of Vehicles
Case
F225
F225-B
F130
F130-B
Vehicle positions
Passenger
vehicles
Large-size
vehicles
Bus
F225-L
Left of fire source point
59
7
0
F225-R
Right of fire source point
59
7
0
F225-B-L
Left of fire source point
57
6
1
F225-B-R
Right of fire source point
57
6
1
F130-L
Left of fire source point
33
4
0
F130-R
Right of fire source point
85
9
0
F130-B-L
Left of fire source point
31
3
1
F130-B-R
Right of fire source point
83
9
1
3.2. Simulation results
Table 4 and Figure 2 shows the simulation results for case F225 and case F225-B. For case
F225-L and case F225-B-L, the fire source point is 225m and the vehicles are arranged from
the tunnel entrance to the fire source point, and the results show no NPRH being affected by
the pressure difference Δ and configuration of vehicles. In case F225-R and case F225-B-R,
vehicles are located on the right side of the fire source point, NPRH break out in these cases
according to the increase of the pressure difference Δ. This result shows that tunnel users are
exposed to smoke by a tunnel fire which has occurred and it flows from the fire in the
7thInternational Conference ‘Tunnel Safety and Ventilation’ 2014, Graz
- 136 direction tunnel users evacuate, so as a result, tunnel users cannot evacuate. In the case of an
arrangement including a bus (case F225-B-R), NPRH are drastically increased.
Table 4: NPRH of fire source point 225m
NPRH [people]
Case
F225
Vehicle positions
Δ 5 Pa
Δ 10 Pa
[0 m/s]
[0.85 m/s]
[1.2 m/s]
Average numbers
of people in a tunnel
F225-L
Left of fire source point
0
0
0
89
F225-R
Right of fire source point
0
0.54
12.89
89
0
0.54
12.89
178
Total
F225-B
Δ 0 Pa
F225-B-L
Left of fire source point
0
0
0
135
F225-B-R
Right of fire source point
0
26.38
53.39
135
0
26.38
53.39
270
Total
Figure 2: The results of fire source point 225m
Figure 3 shows the simulation results at Δ 5 Pa and Figure 4 shows results at Δ10 Pa.
t [min]
t [min]
The x-axis represents the distance from the tunnel entrance and the y-axis represents the
elapsed time after the start of the simulation. In the figure, the green part indicates where
smoke density is Cs 0.4 [1/m] at 1.5m from the road surface and the black-line shows the state
of tunnel users’ evacuation.
x [m]
Figure 3: The results of case F225 Δ5 Pa
x [m]
Figure 4: The results of case F225 Δ10 Pa
7thInternational Conference ‘Tunnel Safety and Ventilation’ 2014, Graz
- 137 Table 5, Figure 6 and Figure 7 shows the simulation from the tunnel entrance to the fire
source point 130m from the tunnel entrance. NPRH trends differ between cases of fire source
points 225m and 130m. For case F130-B-L, vehicles are arranged from the tunnel entrance to
the fire source point and include a bus, and the results show there are NPRH in all cases. The
factors probably are the exposure of tunnel users to smoke backlayering and the presence of a
bus near the fire source point. However, owing to the smoke backlayering moving to the
tunnel exit, NPRH decreased as the pressure difference Δ increased. With case F130-L and
case F130-R, a trend similar to the source point 225m is indicated.
Table 5: NPRH of fire source point 130m
NPRH [people]
Case
F130
Vehicle positions
Δ 0 Pa
Δ 5 Pa
Δ 10 Pa
[0 m/s]
[0.85 m/s]
[1.2 m/s]
F130-L
Left of fire source point
0
0
0
49
F130-R
Right of fire source point
0
0.04
10.24
128
0
0.04
10.24
177
Total
F130-B
Average number
of people in a
tunnel
F130-B-L
Left of fire source point
27.65
23.98
9.19
95
F130-B-R
Right of fire source point
0
0.04
37.76
175
27.65
24.02
46.95
270
Total
t [min]
t [min]
Figure 5: The results of fire source point 130m
Bus
x [m]
Bus
Figure 6: The results of case F130-B, Δ5 Pa
Bus
x [m]
Bus
Figure 7: The results of case F130-B, Δ10 Pa
7thInternational Conference ‘Tunnel Safety and Ventilation’ 2014, Graz
- 138 4.
VALIDITY OF TUNNEL DISASTER PREVENTION SYSTEM
As mentioned above, if there is a bus in the tunnel, NPRH tend to increase. Therefore, this
section determined the effect of an emergency exit. In the tunnel, a bus is leeward of the fire
source point 225m; and the interval between the fire source point and the bus is 10m. The
distances from the bus to an emergency exit are 50m, 75m, 100m, 125m, 150m and 175m.
Figure 8 is a schematic view of these conditions. The pressure difference Δ is 0 Pa.
Tunnel
entrance
150m, -4%
Tunnel
exit
150m, 0%
150m, 4%
225m
Emergency exit
10m
50m
75m
100m
125m
150m
175m
Figure 8: Arrangement of emergency exits [225m]
Figure 9 shows the simulation results for smoke density distribution in the x-z plane across
the tunnel model.
4.5
4.5
4.5
z
Height [m]
4.5
x
4.5
t = 120 second
t = 240 second
t = 360 second
t = 480 second
t = 600 second
Distance from the tunnel entrance [m]
Figure 9: Smoke density distribution in the x-z plane across the tunnel model
Figure 10 indicates the simulation results for the effect of the emergency exit. For the distance
of 50m and 75m, the emergency exit reduces NPRH.
7thInternational Conference ‘Tunnel Safety and Ventilation’ 2014, Graz
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Figure 10: Effectiveness of the emergency exit, Δ 0Pa
5.
CONCLUSIONS
This study quantitatively revealed factors affecting safety for tunnel users when a tunnel fire
has occurred in a relatively short road tunnel. The factors affecting safety of tunnel users are
as follows.
•
•
•
•
A natural wind flows in the direction the tunnel users evacuate: tunnel users are on the
leeward of the fire source point, and smoke catches up to tunnel users.
The tunnel users are exposed to the smoke spread by backlayering.
When a bus is in the tunnel, especially if the bus is close to a fire source point.
Under the study conditions, with the distance of 50m and 75m, the emergency exits
effectively reduce NPRH.
6.
REFERENCE
[1]
PIARC Committee on Road Tunnels (C3.3), Operational Strategies for Emergency
Ventilation, 2011.
[2]
Y.Kunikane, N.Kawabata, K.Takekuni, A.Shimoda, “Heat Release Rate Induced by
Gasoline Pool Fire in a Large-Cross-Section Tunnel”, 4th International Conference,
Tunnel Fires, in Basel, p387-396, (2002. 12. 2-4)
[3]
N.Kawabata, Y.Kunikane, K.Takekuni, A.Shimoda, “Numerical Simulation of Smoke
Descent in a Tunnel Fire Accident”, 4th International Conference, Tunnel Fires, in
Basel, p357-366, (2002. 12. 2-4)
[4]
M. Seike, N. Kawabata and M. Hasegawa, Study of Assessment of Fire Safety in a
Road Tunnel by Evacuee’s Behavior based on Smoke Behavior by 3-D CFD Analysis,
Advanced Research Workshop Evacuation and Human Behavior in Emergency
Situations, Santander, pp.111-125, 2011.
7thInternational Conference ‘Tunnel Safety and Ventilation’ 2014, Graz